This invention provides a select fire mechanism for an M240 automatic rifle that permits the rifle to be operated to be either fully automatic or semi-automatic. The typical M240 is a gas operated, air cooled automatic firearm. Expanding gases from the ignition of powder in the cartridge furnishes the energy for the operation or cycling of the rifle. Immediately after firing, as the bullet traverses the barrel and passes an internal gas port prior to exiting from the muzzle, the live gases expand through the gas port into the gas tube or cylinder to impinge against the head of a piston. In so doing, the piston within the gas cylinder is forced rearwardly, toward the buttstock, driving an operation rod assembly (herein “op rod assembly”) which cycles a combination of internal components while compressing a recoil spring that effects the return movement.
The op rod assembly carries the firing pin and a bolt link pivotally secures the op rod assembly to the bolt lock. As the op rod assembly begins to travel rearwardly under the influence of the expanding gases resulting from the firing of a cartridge, the bolt link rotates and pulls the bolt lock up out of its cradle in the receiver, thereby unlocking the bolt from the receiver. Once free from the mating receiver surfaces, the bolt travels rearwardly with the bolt link and op rod assembly as it continues its rearward travel. In so doing, several additional operations continue. Cartridges for the M240 are belt fed, and the bolt carries a roller that engages a feed lever to feed cartridges into the receiver in a known manner, during this rearward travel. The op rod assembly contacts a buffer in the buttstock, halting its rearward movement, and immediately begins return travel in response to the compression of the recoil spring. Continued forward movement is then a function of the position of the trigger.
Assuming the trigger remains depressed, the sear will not engage the sear ledge on the underside of the op rod assembly and thus the op rod assembly will continue forward under the influence of the recoil spring. During the forward movement, the bolt strips a cartridge from the belt for chambering. Because they are associated with the op rod assembly, the bolt, bolt lock and bolt link also move forward until the very last increment of movement. The bolt moves until the cartridge is seated, which limits the forward movement of the bolt so that the bolt link is caused to rotate downward, lowering the bolt lock into locking engagement with the receiver, at which stage, the bolt cannot move rearwardly. Directly thereafter, the firing pin carried by the op rod assembly is forced to strike the primer of the chambered cartridge, thereby firing it whereby the above firing sequence is repeated in response to sustained depression of the trigger or until the ammunition is exhausted.
Repeat of this cycle is dependent upon the trigger remaining in the fired position. If the operator releases the trigger, the sear is immediately raised into the path of the sear ledge on the reciprocating op rod assembly. In rearward travel, the op rod assembly can push the sear down to pass over it. However, upon the return, the sear is raised by a spring and engages the sear ledge, halting continued forward movement of the op rod assembly. When this occurs, the action remains opened, and no new round is loaded. Accordingly, the M240 rifle is one which fires from the open bolt and, as soon as the trigger is again pulled, the sear is pulled out of engagement with the sear ledge, allowing the op rod assembly to move forward causing all of the foregoing operations. The bolt closes, taking with it a fresh round from the belt, which is simultaneously chambered and instantaneously thereafter fired, as the foregoing sequence is allowed to repeat.
Because fully automatic fire is not always needed and not always desired, the prior art has provided means for converting M240 rifles to fire only semi-automatically. An example of one such means is provided in co-pending U.S. patent application Ser. No. 12/411,026. The conversion shown therein is quite permanent, such that the M240, once converted to semi-automatic fire, is not readily converted back to fully automatic fire. It is believed there is a need in the art for an M240 rifle that includes a select fire mechanism permitting the rifle to be selectively operated as a fully automatic firearm or a semi automatic firearm.